The desire to increase the accuracy of over current protection (OCP) trip points has become of increased importance within the semiconductor industry. OCP measurement accuracy has advanced to the point where circuit offsets are no longer the dominant factor in the OCP error stack up. Even with improvements to OCP performance, it is still not acceptable to many users who find that existing OCP solutions have significant variations and are only usable for short current protection.
The difference between a design with over current protection and a design with short current protection is important. In a design with over current protection, all critical specifications are met up to the point the regulator detects an over current event. One critical specification in this regard is the maximum operating temperature of the voltage regulator. If an OCP trip point accuracy varies between 11 amps and 18 amps as illustrated in FIG. 1, the voltage regulator would be rated for a 10 amp load yet it must be designed so that it can safely operate at 18 amps without a thermal failure. Other failure modes, such as inductor saturation, also must be designed for the higher current level. This requires a designer to build extra margin and cost into their designs. As a result, users see an increase in over current protection accuracy as a real method to reduce their costs.
One present solution to this problem is for users to simply state that they have short current protection, where protection is only provided for low impedance shorts that are detected before a thermal failure occurs. While this works at times for low impedance shorts, intermediate shorts will not be detected prior to the regulator failing from thermal overstress. There is a school of thought which believes that voltage regulators experiencing thermal overstress will increase their temperature and will naturally lower their OCP trip points. FIG. 1 illustrates tests in this manner to exploit this, and even with this affect, a substantial variation in OCP trip points over temperature can be seen.
Prior art voltage regulators use a fixed current source through a fixed resistor to program the over current protection limit. In these cases, there is a large variation on the OCP trip point as a function of input voltage and ambient temperature (see FIG. 1). Additional prior art implementations use a programmable resistor whose values change with temperature to compensate the OCP trip point. These designs typically utilize an NTC or PTC thermistor as appropriate. These solutions have not obtained wide industry acceptance as external thermistors are expensive and their temperature characteristics do not perfectly match the OCP trip point temperature variations for which they are compensating. The solutions also do not compensate for other factors that affect the OCP trip point.
Thus, there is a need for a voltage regulator with a constant OCP threshold as a function of temperature, input voltage and gate voltage. These variables are considered significant first and second order causes for variation in the OCP trip point.